In manufacturing solar cells for civilian applications, at present, it is an important task to reduce the manufacturing cost and, in general, a method of producing a solar cell by the following steps is widely used. The details of the method are, for example, as follows.
First, there is prepared a p-type silicon substrate which is obtained by slicing, based on a multiwire method, a single-crystalline silicon ingot produced by a Czochralski (CZ) method or a polycrystalline silicon ingot produced by a casting method. Next, damages given to the substrate surface by the slicing are removed by an alkaline solution and, thereafter, a microscopic rugged pattern (texture) with a maximum height of about 10 μm is formed in both a light-receiving surface and a back surface. Subsequently, a dopant is thermally diffused into the substrate by one of various methods to form an n-type diffusion layer. Further, TiO2 or SiN is deposited on the light-receiving surface in a thickness of, for example, about 70 nm to form an antireflective film. Next, a paste for a back-surface electrode that contains aluminum as a main ingredient is printed on the whole area of the back surface, followed by firing to form a back-surface electrode. On the other hand, a light-receiving surface electrode is formed by printing on the light-receiving surface a paste for a light-receiving surface electrode that contains silver as a main constituent in a comb pattern with a width of, for example, about 100 to 200 μm, followed by firing.
Such a technique is excellent in that various enhancing effects on solar cell characteristics such as energy conversion efficiency are attendantly realized, notwithstanding the technique involves a minimum number of steps necessary for fabricating the device. For instance, the thermal diffusion of the dopant in forming the diffusion layer in the substrate has a working effect of improving the diffusion length of small-number carriers in the bulk by a gettering action. In addition, the firing of aluminum printed on the back surface, during formation of the back-surface electrode, can simultaneously result in both formation of the electrode and formation of a p+ high-concentration layer having a BSF (Back Surface Field) effect on the back surface. Furthermore, the antireflective film has both an optical effect (reduction in reflectance) and a function of reducing the recombination velocity of carriers generated near the silicon surface.
By the minimum required number of steps and some useful effects as above-mentioned, it is promised to manufacture civilian-use solar cells at lower costs than before.
Even with the above-mentioned excellent technique, however, the conversion efficiency of a solar cell utilizing a single-crystalline silicon substrate, for example, reaches an upper limit of about 16%, and a further considerable improvement in conversion efficiency cannot be expected. In fact, in order to sufficiently lower the contact resistance of a light-receiving surface electrode, the surface concentration of the dopant such as phosphorus in the diffusion layer should be about 2.0 to 3.0×1020 cm−2. When the substrate surface is made to contain the dopant in such a high concentration, the surface level becomes very high, so that carrier recombination near the light-receiving surface is accelerated. Therefore, short-circuit current and open-circuit voltage are restricted and, consequently, the conversion efficiency reaches an upper limit.
In view of this, there has been proposed a method for improving conversion efficiency by lowering the surface concentration in the diffusion layer formed at the light-receiving surface. For example, a proposal relating to this method has been known as disclosed in U.S. Pat. No. 6,180,869 (Patent Document 1). According to the document, low-ohmic contact can be formed even when the surface concentration in the diffusion layer is about 1.0×1020 cm−2 or below. This is realized by addition of a compound containing a dopant to the vicinity of a silver filler contained in the paste for electrode. As a result of this approach, a high-concentration layer of the dopant is formed beneath the electrode upon firing of the electrode.
However, by the method in which the dopant-containing compound is thus added near the silver filler contained in the paste for electrode, it is impossible to form a contact between the diffusion layer and the electrode in a stable manner. Therefore, there is a problem that the solar cell obtained is low in fill factor and in reliability.
Besides, as a method of enhancing conversion efficiency by forming a high-concentration diffusion layer (emitter layer) containing a dopant in a high concentration only beneath an electrode while lowering the surface concentration in the diffusion layer in other areas of a light-receiving surface, that is, by forming a two-stage (two-level) emitter, there has been known, for example, “Photo-electric conversion device and process for producing the same” as disclosed in JP-A 2004-273826 (Patent Document 2). This method is a modification, from electroplating method to screen printing method, of a process for forming an electrode in a solar cell with an embedded type electrode that has been known from JP-A 8-37318 (Patent Document 3) and JP-A 8-191152 (Patent Document 4). It is said to be possible by this method to facilitate production control and to lower the production cost.
However, in order to obtain the two-stage emitter in the process for producing a solar cell with an embedded type electrode as described in Patent Document 2, a heat treatment for forming an n-type diffusion layer is followed by a heat treatment for forming a high-concentration n-type diffusion layer. Therefore, it is necessary to carry out heat treatment at least twice, which leads to complicated working steps and an increase in production cost.
In addition, as another method for enhancing conversion efficiency by forming a two-stage emitter, there has been known, for example, “Process for producing a solar cell” as disclosed in JP-A 2004-221149 (Patent Document 5). It is proposed in this document that individual coatings with a plurality of coating agents by an ink jet system are carried out simultaneously so as to create regions different in dopant concentration and/or dopant species by a simple step.
When phosphoric acid or the like is used as a dopant in such an ink jet system, however, a countermeasure against corrosion is needed, which leads to a complicated system and intricate maintenance. In addition, even if the coating agents different in dopant concentration and/or dopant species are individually coated by ink jet, diffusion by one run of heat treatment results in that a desired concentration difference cannot be obtained due to auto-doping.
Furthermore, as a further method for enhancing conversion efficiency by forming a high-concentration diffusion layer only beneath an electrode and lowering the surface concentration in the diffusion layer in other areas of a light-receiving surface, there has been known, for example, “Process for producing a solar cell” as disclosed in JP-A 2004-281569 (Patent Document 6).
In the process according to Patent Document 6, however, it is necessary to conduct diffusion heat treatment twice for forming a low-concentration diffusion layer and a high-concentration diffusion layer, so that the process is not simple. If the heat treatment is conducted only once, taking this drawback into account, the dopant concentration becomes high also in other areas than the area beneath the electrode of the light-receiving surface due to auto-doping. Consequently, the cell obtained would not show a high conversion efficiency.
Taking the foregoing into consideration, JP-A 2006-310373 (Patent Document 9) proposes a process wherein a first coating agent containing phosphoric acid and a second coating agent containing diphosphorus pentoxide are simultaneously applied to a p-type substrate by screen printing, followed by thermal diffusion, so as to simultaneously form a high-concentration diffusion layer and a low-concentration diffusion layer.
This ensures that the formation of a two-stage emitter that has been intricate due to formation of a diffusion mask or the like is made to be very simple, resulting in a lowering in production cost. Besides, since a sufficient surface concentration is maintained in the high-concentration diffusion layer, a low-ohmic contact can be formed easily. Consequently, a high-performance solar cell can be produced while maintaining the production yield at a high level.
Thus, the merit of using the screen printing method in constructing the two-stage emitter resides in that an arbitrary pattern can be formed easily, that a diffusion agent with a high dopant concentration can be applied to a substrate surface in a uniform thickness by one time of printing, and that dopant diffusion can be efficiently performed by a high-concentration phosphorus glass layer in the subsequent heat treatment. Besides, another merit lies in that a high-concentration diffusion layer can be formed by the printing treatment and the heat treatment which are each completed in a short time.
The application of the paste for diffusion that is used for forming the diffusion layer may thus be conducted by a screen printing method as in Patent Document 9, and may also be conducted by a spin coating method. The spin coating method is preferable for forming a layer on a surface in a uniform thickness, but is very wasteful because much of the material is scattered at the time of spinning. Examples of the coating liquid for diffusion that is used in spin coating include a coating liquid for phosphorus diffusion described in JP-A 2007-53353 (Patent Document 7) and a coating liquid for boron diffusion described in JP-A 2007-35719 (Patent Document 8).
In contrast to such a spin coating method, the screen printing method ensures, as in Patent Document 9, that a large amount of a diffusion agent can be layered on a substrate surface through a speedy printing treatment, without wasting the material.
As described also in Patent Document 7, however, a coating agent for diffusion is generally composed of a water-soluble phosphorus paste containing a phosphorus compound, a water-soluble polymer compound, and water, and the viscosity of the water-soluble paste is susceptible to variations by ambient environments such as humidity. Moreover, upon continuous printing, even a water-soluble phosphorus paste conditioned in viscosity undergoes thickening due to dehumidification after a large number of times of printing, with the result of clogging of screen meshes. Thus, with a water-soluble phosphorus paste, it has been impossible to stably carry out continuous printing for a long time or a large number of times.